Images of child pornography are not protected under First Amendment rights, and are illegal contraband under federal law. Section 2256 of Title 18, United States Code, defines child pornography as any visual depiction of sexually explicit conduct involving a minor (someone under 18 years of age). Visual depictions include photographs, videos, digital or computer generated images indistinguishable from an actual minor, and images created, adapted, or modified, but appear to depict an identifiable, actual minor.
If they allowed so much sexual assault material to continue to be shared on their platform they would inevitably run into trouble.